This invention relates to HSDPA in a wireless communication system. More particularly, this invention prevents the losing synchronization for UM RLC when the communication switches to use a smaller TBS. size.
The High Speed Downlink Packet Access (HSDPA) between UTRAN and UEs is based on techniques, such as HARQ (Hybrid Automatic Repeat Request) transmission and reception protocol used on HS-DSCH (High Speed Downlink Shared Channel) and adaptive modulation to achieve high throughput, reduce delay and high peak rates. The new functionality of hybrid ARQ and HSDPA scheduling are included in the MAC layer as shown in FIGS. 1, 2 and 4.
At the transmitter side—an UTRAN, the HARQ functions are included and controlled in a new entity called MAC-hs located in Node B as shown in FIG. 5. Such HARQ functions include a scheduler, HARQ entity, and HARQ process. A scheduler schedules MAC-hs SDUs for all connected UEs within a cell. The scheduler determines the HARQ Entity and the queue to be serviced and services priority queues. Meantime, the scheduler will indicate each sending MAC-hs PDU with a Queue ID and a Transmission Sequence Number (TSN) to the HARQ entity; the scheduler also schedules new transmissions, retransmission, determines a suitable redundancy version for each transmitted and retransmitted MAC-hs PDU and indicates the redundancy version to lower layer.
Next, there is one HARQ entity assigned to an UE in the UTRAN. The HARQ entity sets both the Queue ID and the TSN in transmitted MAC-hs PDUs. The TSN will be incremented for each subsequent transmitted MAC-hs PDU on a HS-DSCH. In addition, the HARQ entity determines a suitable HARQ process to service the MAC-hs PDU and sets the HARQ process identifier accordingly. Meantime, the UTRAN's HARQ process sets the New Data Indicator for the transmitted MAC-hs PDUs. The system sets the New Data Indicator to the value “0” for the first MAC-hs PDU transmitted by a HARQ process. The system increments the New Data Indicator by one for each subsequent transmitted MAC-hs PDU containing new data, while it does not increment the New Data Indicator if the MAC-hs PDU is a retransmission one.
Furthermore, the MAC layer uses MAC-hs PDU as the basic data structure to communicate with its upper or below layers or between the transmitter and the receiver. FIG. 6 illustrates the detail format of such MAC-hs PDU where a regular MAC-hs PDU includes a MAC-hs header, a plurality of MAC-hs SDUs and optional padding field. The system uses a variable size of PDU for transmission. There is a trade-off between using the larger size of Transport Block Set (TBS.) vs. the degree of robust. The system can transmit a MAC-hs PDU with its maximum size, which includes maximum number of MAC-hs SDUs (Service Data Units) in one PDU under the environment of requiring the least degree of robust. When the transmitting condition changes, the size of PDU, which in turn means the number of SDUs carried in one PDU, has to be reduced to ensure a more robust transmission.
At the receiver, a UE, to support the HARQ protocol, the HSDPA requires the support of functionality of HARQ Entity, HARQ process, reordering buffers and disassembly Entity. As shown in FIGS. 4 and 5, one UE has only one HARQ entity while supports a number of parallel HARQ processes. The HARQ Entity processes the HARQ process identifiers received on HS-SCCH. And based on the HARQ process identifier, the HARQ entity allocates the received MAC-hs PDU to the corresponding HARQ process.
At the same time, if the New Data Indicator of the new received MAC-hs PDU has been incremented compared to the value in the previous received transmission in this HARQ process or this is the first received transmission in the HARQ process, the UE shall replace the data currently in the soft buffer for this HARQ process with the received data. On the other hand, if the New Data Indicator is identical to the value used in the previous received transmission in the HARQ process, the UE shall combine the received data with the data currently in the soft buffer for this HARQ process. Later, if the data in the soft buffer has been successfully decoded and no error was detected, the UE delivers the decoded MAC-hs PDU to the reordering entity and generates a positive acknowledgment (ACK) of the data in this HARQ process. Otherwise, the UE should generate a negative acknowledgment (NAK) of the data in this HARQ process and report it to the transmitter. Moreover, the UE's HARQ process processes the Queue ID of the received MAC-hs PDUs and forwards the received MAC-hs PDUs to the reordering queue distribution entity. The reordering queue distribution function routes the MAC-hs PDUs to the correct reordering buffer based on the Queue ID.
Later, the reordering entity reorders received MAC-hs PDUs according to the received TSN. MAC-hs PDUs with consecutive TSNs are delivered to the disassembly function upon reception. MAC-hs PDUs are not delivered to the disassembly function if MAC-hs PDUs with lower TSN are missing. There is one reordering entity for each Queue ID configured at the UE. The reordering entity can use a Timer based stall avoidance mechanism or a Window based stall avoidance mechanism or both of them to manage its reordering buffer. Through the disassembly function, the UE shall remove any padding bits if present and the MAC-hs header, then deliver the MAC-d PDUs in the MAC-hs PDU to MAC-d.
Meantime, the system uses several ways to handling its most frequent encountered transmission errors: First, a NACK is detected as an ACK. The NW (Network) starts afresh with new data in the HARQ process where the data block is discarded in the NW and is lost. Retransmission of the discarded data block is left up to higher layers. Second, an ACK is detected as a NACK. If the network retransmits the data block, the UE will re-send an ACK to the network. In this case the transmitter sends an abort indicator by incrementing the New Packet Indicator, the receiver will continue to process the data block as in the normal case. Third, the NW implements an automatic default NACK return if the system sets up a threshold to detect lost status message. And last, if a CRC error happened on the HS-SCCH, UE receives no data and sends no status report. If the absence of the status report is detected, NW can retransmit the data block.
Usually, the system uses a tradeoff scheme between the size of Transport Block Set (TBS.) and the degree of robust for data transmission. In the least robust modulation and coding scheme (MCS) condition within the TTI (Transmission Timing Interval), the system uses the largest TBS. size of MAC-hs PDU to maximum data transmission rate. When radio condition changes while a more robust MCS is required for successful transmission, instead of using the largest TBS. size of a maximum date rate, the system has to switch to use a smaller TBS. size for a slower data rate within the TTI.
Nevertheless, according to the prior art, every time the system switches from operating at the higher data rate with a larger TBS. size to use a smaller size of MAC-hs PDU for a more robust MCS requirement, all outstanding MAC-hs PDUs in HARQ processes that have not been positively acknowledged will be discarded. The discarding of the MAC-hs PDUs definitely causes system inefficiency and latency that requires effects from higher layers of the transmitter to recovery these discarded data blocks.
The problems of the prior art can be further illustrated in the following two examples. First, assume that a UE's HARQ entity has six HARQ processes and is using the largest MAC-hs PDU size for transmission. Process 1 to 6 transmits MAC-hs PDUs with TSN 0 to 5 to UE respectively. After the first transmission of each process, the transmitter receives positive acknowledgments for MAC-hs PDUs with TSN=0, 2, 4 and 5 and negative acknowledgments for MAC-hs PDUs with TSN=1 and 3. Before starting the second transmission of each process, due to the change of radio conditions, the system has to use a smaller TBS. size for MAC-hs PDU size. According to the prior art, MAC-hs PDUs with TSN=1 and 3 will be discarded, so are dozens of RLC PDUs contained in these two PDUs. If MAC-hs PDUs with TSN=1 and 3 contain RLC UM PDUs, these RLC UM PDUs are lost forever.
Second, again assume that the largest MAC-hs PDU size is used in the MAC-hs PDU and the HARQ entity of an UTRAN allocates six HARQ processes to transmit data to one UE. Each HARQ process is transmitting one MAC-hs PDU and none of them has not received any acknowledgment for its transmitted MAC-hs PDU. Due to the change of the radio conditions, now the MCS must switch to use a more robust MCS that means to use a smaller size of MAC-hs PDU for transmission. According to the prior art, the six MAC-hs PDUs transmitted without positively acknowledged must be all discarded because their sizes does not conform to the new MAC-hs PDU size. If the six pending MAC-hs PDUs include more than 128 consecutive UM PDUs all sent by the same UM RLC entity, the HFN synchronization of the peer UM entities will be lost. Consequently, all the following UM PDUs will not be deciphered correctly. The prior art has no mechanism in the RLC layer to handle this serious problem.